Disk drive having built-in self-test system for characterizing performance of the drive

Abstract

A disk drive has a normal mode of operation and a built-in self-test (BIST) mode of operation for producing a sequence of channel metrics {Γ n }. The disk drive includes a recording surface having a plurality of bit cells and a transducer for reading the plurality of bit cells to produce a noise-corrupted read signal. The disk drive further includes means responsive to the noise-corrupted read signal for generating a sequence of observed samples {y n }, the sequence of observed samples {y n } forming a sequence of observed-sample subsequences {Y n }. An expected sample generator operates during the BIST mode of operation to provide a sequence of expected samples {w n }, the sequence of expected samples forming a sequence of expected-sample subsequences {W n }. A channel metrics Γ n computation system computes a sequence of channel metrics {Γ n }. Each channel metric Γ n is a function of a distance determined from one of the observed-sample subsequences Y n to the corresponding expected-sample subsequence W n . Each channel metric Γ n is independent of the earliest observed sample in every prior observed-sample subsequence Y n and the earliest expected sample in every prior expected-sample subsequence W n .

We claim:  
     
       1. A disk drive having a normal mode of operation and a built-in self-test mode of operation for producing a sequence of channel metrics {Γ n }, the disk drive comprising: 
       a recording surface having a plurality of bit cells;  
       a transducer for reading the plurality of bit cells to produce a noise-corrupted read signal;  
       means responsive to the noise-corrupted read signal for generating a sequence of observed samples {y n }, the sequence of observed-samples {y n } forming a sequence of observed-sample sequences {Y n }, each observed sample sequence Y n  having an earliest observed sample and a latest observed sample;  
       means operative during the built-in self-test mode of operation for providing a sequence of expected samples {w n }, the sequence of expected samples forming a sequence of expected-sample subsequences {W n }, each expected-sample subsequence W n  having an earliest expected sample and a latest expected sample;  
       computation means for computing a sequence of channel metrics {Γ n }, each channel metric Γ n  being a function of a distance determined from one of the observed-sample subsequences Y n  to the corresponding expected-sample sequence W n , each channel metric Γ n  being independent of the earliest observed sample in every prior observed-sample subsequence Y n  and the earliest expected sample in every prior expected-sample subsequence W n ;  
       means for computing a mean μ Γ  of the channel metrics Γ n ;  
       means for computing a standard deviation σ Γ  of the channel metrics Γ n ; and  
       means for computing a ratio of the mean μ Γ  to the standard deviation σ Γ , the ratio (μ Γ /σ Γ ) corresponding to a signal to noise ratio.  
     
     
       2. The disk drive of claim  1  further comprising means for estimating a bit error rate of the disk drive from the ratio (μ Γ /σ Γ ). 
     
     
       3. The disk drive of claim  2 , wherein the means for estimating the bit error rate comprises a means for computing Q(μ Γ /σ Γ ) where Q is a Gaussian Q function. 
     
     
       4. A method for computing a sequence of channel metrics {Γ n } for characterizing the performance of a disk drive, the method comprising the steps of: 
       reading a plurality of bit cells stored on a recording surface in the disk drive to produce a noise-corrupted read signal;  
       generating a sequence of observed samples {y n }, the sequence of observed-samples {y n } forming a sequence of observed-sample sequences {Y n }, each observed sample sequence Y n  having an earliest observed sample and a latest observed sample;  
       providing a sequence of expected samples {w n }, the sequence of expected samples forming a sequence of expected-sample subsequences {W n }, each expected-sample subsequence W n  having an earliest expected sample and a latest expected sample;  
       computing a sequence of channel metrics {Γ n }, each channel metric Γ n  being a function of a distance determined from one of the observed-sample subsequences Y n  to the corresponding expected-sample sequence W n , each channel metric Γ n  being independent of the earliest observed sample in every prior observed-sample subsequence Y n  and the earliest expected sample in every prior expected-sample subsequence W n ; and  
       computing a mean μ Γ  of the channel metrics Γ n ;  
       computing a standard deviation σ Γ  of the channel metrics Γ n ;  
       computing a ratio of the mean μ Γ  to the standard deviation σ Γ , the ratio (μ Γ /σ Γ ) corresponding to a signal to noise ratio.  
     
     
       5. The method of claim  4  further comprising the step of estimating a bit error rate of the disk drive from the ratio (μ Γ /σ Γ ). 
     
     
       6. The disk drive of claim  5 , wherein the step of estimating the bit error rate comprises the step of computing Q(μ Γ /σ Γ ) where Q is a Gaussian Q function. 
     
     
       7. A method for estimating a bit error rate for a disk drive, the method comprising the steps of: 
       reading a plurality of bit cells stored on a recording surface in the disk drive to produce a noise-corrupted read signal;  
       generating a sequence of observed samples {y n } responsive to the noise-corrupted read signal, the sequence of observed samples {y n } forming a sequence of observed-sample subsequences {Y n }, each observed-sample subsequence Y n  having an earliest observed sample and a latest observed sample;  
       providing a sequence of expected samples {w n }, the sequence of expected samples forming a sequence of expected-sample subsequences {W n }, each expected-sample subsequence W n  having an earliest expected sample and a latest expected sample;  
       computing a sequence of channel metrics {Γ n }, each channel metric Γ n  being a function of a distance determined from one of the observed-sample subsequences Y n  to the corresponding expected-sample subsequence W n , each channel metric Γ n  being independent of the earliest observed sample in every prior observed-sample subsequence Y n  and the earliest expected sample in every prior expected-sample subsequence W n ;  
       computing a mean μ Γ  of the channel metrics {Γ n };  
       computing a standard deviation σ Γ  of the channel metrics {Γ n };  
       computing a ratio of the mean μ Γ  to the standard deviation σ Γ , the ratio (μ Γ /σ Γ ) corresponding to a signal to noise ratio; and  
       estimating the bit error rate from the ratio (μ Γ /σ Γ ).  
     
     
       8. The disk drive of claim  7 , wherein the step of estimating the bit error rate comprises the step of computing Q(μ Γ /σ Γ ) where Q is a Gaussian Q function. 
     
     
       9. A disk drive having a normal mode of operation and a built-in self-test mode of operation for producing a sequence of channel metrics {Γ n }, the disk drive comprising: 
       a recording surface having a plurality of bit cells;  
       a transducer for reading the plurality of bit cells to produce a noise-corrupted read signal;  
       a sampler responsive to the noise-corrupted read signal for generating a sequence of observed samples {y n }, the sequence of observed-samples {y n } forming a sequence of observed-sample sequences {Y n };  
       an expected sample generator operative during the built-in self-test mode of operation for providing a sequence of expected samples {w n }, the sequence of expected samples forming a sequence of expected-sample subsequences {W n };  
       channel metrics computer for computing a sequence of channel metrics {Γ n }, each channel metric Γ n  being a function of a distance determined from one of the observed-sample subsequences Y n  to the corresponding expected-sample sequence W n ; and  
       a ratio computer for computing a mean μ Γ  of the channel metrics Γ n , a standard deviation σ Γ  of the channel metrics Γ n , and a ratio of the mean μ Γ  to the standard deviation σ Γ , the ratio (μ Γ /σ Γ ) corresponding to a signal to noise ratio.  
     
     
       10. The disk drive of claim  9 , furthers comprising a bit error rate estimator for estimating a bit error rate of the disk drive by computing Q(μ Γ /σ Γ ) where Q is a Gaussian Q function. 
     
     
       11. A disk drive having a normal mode of operation and a built-in self-test mode of operation for producing a sequence of channel metrics {Γ n }, the disk drive comprising: 
       a recording surface having a plurality of bit cells;  
       a transducer for reading the plurality of bit cells to produce a noise-corrupted read signal;  
       a sampler responsive to the noise-corrupted read signal for generating a sequence of observed samples {y n }, the sequence of observed-samples {Y n } forming a sequence of observed-sample sequences {Y n };  
       an expected sample generator operative during the built-in self-test mode of operation for providing a sequence of expected samples { w n }, the sequence of expected samples forming a sequence of expected-sample subsequences {W n };  
       channel metrics computer for computing a sequence of channel metrics {Γ n }, each channel metric Γ n  being a function of a distance determined from one of the observed-sample subsequences Y n  to the corresponding expected-sample sequence W n , the channel metrics computer comprising:  
       equation circuitry for implementing a plurality of equations to compute a plurality of different channel metrics Γ n ; and  
       an equation selection matrix for selecting between the different channel metrics Γ n  computed by the equation circuitry, the selection based on a selected number of the expected samples {w n }, the equation matrix generating a selected channel metric Γ n ; and  
       an accumulator for accumulating the selected channel metric Γ n .  
     
     
       12. A method for computing a sequence of channel metrics {Γ n } for characterizing the performance of a disk drive comprising a recording surface having a plurality of bit cells and a transducer for reading the plurality of bit cells to produce a noise-corrupted read signal, the method comprising the steps of: 
       sampling the noise-corrupted read signal to generate a sequence of observed samples {y n }, the sequence of observed-samples {y n } forming a sequence of observed-sample sequences {Y n };  
       generating, during the built-in self-test mode of operation, a sequence of expected samples {w n }, the sequence of expected samples forming a sequence of expected-sample subsequences {W n };  
       computing a sequence of channel metrics {Γ n }, each channel metric Γ n  being a function of a distance determined from one of the observed-sample subsequences Y n  to the corresponding expected-sample sequence W n , the step of computing the channel metrics comprising the steps of:  
       computing a plurality of different channel metrics Γ n ; and  
       selecting between the different channel metrics Γ n  based on a selected number of the expected samples {w n } to generate a selected channel metric Γ n  ; and  
       accumulating the selected channel metric Γ n .